Commercial refrigeration systems are used to maintain a cooled space or a refrigerated product at a desired temperature. Refrigerated cases or rooms are commonly used by grocery stores, restaurants, food distributors, and warehouses of various types. In order to maintain the quality of the refrigerated product while minimizing the cost of refrigeration, it is necessary to keep the temperature of the refrigerated product or case as close to the desired temperature as possible. If the temperature is allowed to rise, the quality or integrity of the product may be jeopardized. If the temperature is kept lower than necessary, energy is wasted and the already high cost of refrigerating the space is increased unnecessarily. For example, it is estimated that as much as two percent more energy is required for each degree the temperature runs below the desired temperature.
A refrigeration system provides a cooling effect by pumping a refrigerant through an evaporator, where the refrigerant changes from a liquid state to a gaseous state. One or more compressors are required to compress the gaseous refrigerant and return it to a liquid state before it passes through an expansion valve and through the evaporator. Because the compressors are the primary electromechanical element in the refrigeration system, they are the focus of most concerns regarding power consumption, cooling efficiency, wear and tear, and expense.
Commercial refrigeration systems are typically controlled by a rack controller. The rack controller is connected to temperature sensors or pressure sensors to determine whether the refrigerated space is too warm, too cool, or just right. Based on input from these sensors, the rack controller determines when one or more compressors should be turned on or off to adjust the cooling capacity of the system. Rack controllers start and stop a compressor by causing power to be applied to or removed from the compressor.
Commercial refrigeration systems typically employ either reciprocating compressors or screw compressors. Each type has its own advantages and disadvantages. For example, reciprocating compressors are available in lower horsepower ranges and are considerably less expensive on a per horsepower basis. However, reciprocating compressors cannot tolerate any liquid refrigerant and cannot overspeed to increase capacity and efficiency. Screw compressors, on the other hand, are smaller, more efficient, and require less maintenance than reciprocating compressors. They can also absorb as much as 40 per cent liquid refrigerant by volume. The primary disadvantage of screw compressors is that they cost approximately twice as much as a comparable size reciprocating compressor. As a result of their cost, those who use screw compressors have a significant incentive to ensure that the compressors are not damaged due to improper operating conditions.
Compressors can be protected from some types of damage by monitoring operating conditions and shutting down the compressor if conditions deviate from a safe operating range. For example, U.S. Pat. No. 5,209,076 describes a control system for preventing damage to a compressor in a refrigeration system. In the described system, a microprocessor monitors the refrigerant pressure, temperature, superheat, oil pressure, and motor current draw during the operation of a compressor and shuts it down automatically under certain conditions. It also provides a reset button to reset the compressor in the event a condition being monitored caused it to shut down.
Because screw compressors are more susceptible to damage resulting from improper operating conditions, it is advantageous to monitor a variety of parameters. Some prior art controllers have been designed to monitor oil flow, discharge temperature, and motor overload conditions while a screw compressor is operating. If one or more of these parameters deviate from a its predetermined operating range, an alarm will be triggered and the compressor will be shut off.
Because safe operation of a screw compressor can be affected by a variety of additional factors, it is advantageous to monitor additional parameters during startup, normal operation, and shut down of the compressor. In the prior art, however, monitoring additional parameters has required a variety of peripheral devices and has resulted in the monitoring and controlling equipment being larger, more complex, and more expensive. In addition, because separate devices are used to monitor different conditions, the processes associated with monitoring operating conditions, controlling the compressor in response to changing conditions, and providing notification or alarms are not well integrated.
It is desirable to fully integrate the monitoring, control, and alarm functions because this allows various situations to be handled in the most effective and efficient manner. For example, some conditions will require that an alarm be sounded and that the compressor be shut down until an operator remedies the failed condition and clears the alarm. In other situations, it may be safe to continue to operate a compressor while sounding an alarm in order to bring a condition to the attention of an operator. In yet another scenario, it may be appropriate to shut down the compressor, sound an alarm, and then attempt to restart the compressor after a predetermined amount of time has passed or after a condition has returned to an acceptable range.
Therefore, there is a need in the art for an improved system and method for monitoring and controlling refrigeration compressors. The improved system and method should provide an integrated facility for monitoring operating conditions, controlling the compressor, and providing notification of fault conditions. The system should monitor relevant conditions and respond to the monitored conditions by providing alarms and by controlling the compressor in a manner that provides maximum compressor availability while reducing the risk of damage to the compressor.